The present invention relates to a magnetic disk drive that implements positioning of a magnetic head using a voice coil motor (abbreviated to VCM hereinafter), and to a speed control method using the back electromotive force of a VCM.
As disclosed in JP-A 11-25625, a VCM back electromotive force sensor is used for speed control during loading and unloading of the magnetic head in a magnetic disk drive system that has a mechanism that moves the magnetic head into an evacuation area on the disk surface when it is not operating, and this is called a load-unload system.
In a well-known method of detecting back electromotive force, a voltage that is proportional to the VCM drive current is subtracted from the VCM coil terminal voltage, as shown in FIG. 2. In this figure, the VCM coil 1 can be modeled, as shown in the diagram, as a series connection of three elements: resistor 4 of resistance value Rm; an inductance 5 of inductance value Lm; and a back electromotive force 6 that should be detected with those. The VCM coil 1 is connected in series with a current sensing resistor 3 to a driver circuit, not shown, and the current 2 is controlled by the driver circuit. In such a circuit, the resistance R1 of resistor 27 and the resistance R2 of resistor 28, which are connected to the operational amplifier 26, take on values that satisfy the relationship Rm=R2/R1×Rs. When the ratio between resistance R3 and resistance R4 of resistors 30 through 33 connected to operational amplifier 29 is G=R4/R3, the detected back electromotive force signal Bemf_h can be expressed by the following equation:Bemf—h=G·(Bemf+sLm·Im)+Vref  (Equation 1)
The principle of measurement according to Equation 1, which is associated with conventional back electromotive force sensors, will be explained in further detail using the block diagram shown in FIG. 9. As explained above, the VCM coil can be modeled by connecting, in series, resistor Rm, inductance Lm and the back electromotive force Bemf, that should be detected. The impedance Z of this VCM coil can be expressed as Rm sLm. Here, s is a differential operator. Block 112 shows the coefficient for conversion from [VCM coil terminal voltage Vs] to [VCM coil current Im], that is, the inverse of the VCM coil impedance. Reference numeral 104 denotes the VCM coil current Im. This current Im can be expressed by multiplying the inverse of the VCM coil impedance and the differential voltage 103 between the VCM terminal voltage Vs 101 and the back electromotive force Bemf 102 produced by movement of the actuator.
Back electromotive force sensors that use conventional technology calculate the voltage drop Vrm 105, caused by VCM coil resistance Rm, from the VCM coil current Im 104. They then determine the back electromotive force signal Bemf_h 106 by subtracting this voltage drop from the VCM terminal voltage 101. In FIG. 9, block 113 represents the coefficient for conversion from [VCM coil current Im] to [current drop Vrm caused by VCM coil resistance], that is the VCM coil resistance Rm.
The following Equation 2 is derived from the above-described theoretical flow.Bemf—h=Vs−Rm·Im=[Bemf+(Rm+sLm)·Im]−Rm·Im=Bemf+sLm·Im  Equation 2)
Equation 1 above is obtained when this theoretical equation is made to correspond to the circuit of the conventional back electromotive force sensor shown in FIG. 2.
Thus, the value of the back electromotive force detected in conventional circuits includes an item that is proportional to the VCM coil current Im differential. In other words, the second item sLm·Im in the parentheses on the right side of Equation 1 includes the differential operator s, which indicates a Laplace transformation. This item is proportional to the VCM coil current Im differential. Accordingly, when the VCM coil current Im changes and directly after it changes, the transient response caused by this differential item means that the correct back electromotive force cannot be detected. Therefore, in a load-unload control system that uses this back electromotive force sensor, the transient response caused by this differential item is set by using a sufficiently large control sample period (for example 700 micro seconds (μs)), thus eliminating this effect.